In process measurements technology and in industrial measurements technology, for measurement of the electrical conductivity of a liquid, conductivity sensors are frequently used, which work according to an inductive or a conductive measuring principle.
From EP 990 894 B1, for example, a conductivity sensor is known, which includes at least two electrodes, which, for measurement purposes, are immersed in a medium. For determining the electrical conductivity of the medium, the resistance or conductance of the electrode measuring path in the medium is determined. In the case of known cell constant, the conductivity of the medium can be ascertained therefrom. For measurement of the conductivity of a measured liquid by means of a conductivity sensor, it is absolutely necessary to bring at least two electrodes in contact with the measured liquid.
In the case of the inductive principle of conductivity determination for process media, sensors are applied, which have a transmitting coil, as well as a receiving coil arranged spaced apart from the transmitting coil. Via the transmitting coil, an electromagnetic alternating field is produced, which acts on charged particles—e.g. ions—in the liquid medium, and brings about a corresponding electrical current flow in the medium. Via this electrical current flow, also on the receiving coil, an electromagnetic field arises, which induces a received signal (induction voltage) in the receiving coil according to Faraday's law of induction. This received signal can be evaluated and used for determining the electrical conductivity of the liquid medium.
Typically, inductive conductivity sensors are constructed as follows: The transmitting and receiving coils are, as a rule, embodied as toroidal coils and comprise a traversing opening contactable by the medium, so that the two coils are flowed around by medium. In the case of excitation, the transmitting coil forms a closed electrical current path extending within the medium, which passes through both the transmitting as well as also the receiving coil. By evaluation of the electrical current or voltage signal of the receiving coil in response to the signal of the transmitting coil, the conductivity of the measured liquid can then be ascertained. The principle as such is established in industrial process measurements technology and is documented in a large number of documents in patent literature, for example, in DE 198 51 146 A1.
Gradiometry with the assistance of a gradiometer is the simultaneous measurement of a gradient using two sensors, which are arranged at a fixed distance relative to one another. Frequently, gradiometers are used for measurement of magnetic fields. In such case, the sensors are generally embodied as magnetometers, in the simplest case as coils. In such case, the measured values are most often subtracted one from the other, in order to detect magnetic field differences between the two sensors. Thus, highly precise measurements of the magnetic field can be produced.
Thus, coils in the arrangement can, as gradiometers, be used as conductivity sensors.
Problematic in the case of gradiometers is that, due to the field configuration, they are affected by all conductive objects in their nearer environment. Especially metal objects, e.g. a containment wall, can act in a disturbing manner on the measurement, since eddy currents are generated in them, which bring about a disturbance signal, which is superimposed on the actual measurement signal.